1. Field of the Invention
The present invention relates to a dispersion comprising a) metallic, metal oxide, metal precursor nanoparticles,
b) a polymeric dispersant (PD) comprising (i) a polyacetal or a polyacetal/polyether backbone, and (ii) an anchor group with affinity for the metallic, metal oxide or metal precursor nanoparticles that is chemically bonded to the polyacetal or polyacetal/polyether backbone, the PD having 95 wt. % decomposition at a temperature below 300° C., c) an optional dispersion medium, and d) a thermally cleavable agent (TCA), characterized in that the TCA is Brönsted acid having a pKa below 4.4 and a boiling point of 100° C. or higher
2. Description of the Related Art
The interest in metallic nanoparticles has increased during the last decades due to their unique properties when compared to the bulk properties of a given metal. For example, the melting point of metal nanoparticles decreases with decreasing particle size making them of interest for printed electronics, electrochemical, optical, magnetic and biological applications.
The production of stable and concentrated metallic nanodispersions that can be coated or printed, for example by inkjet printing, with a high throughput is of great interest as it enables the preparation of electronic devices at low costs.
Usually, the production of metallic nanodispersions is carried out in water or organic solvents by the polyol synthesis methodology (as disclosed in Mat. Chem. Phys. 114, 549-555), by a derivative of the polyol synthesis methodology, or by an in-situ reduction of metallic salts in the presence of various reducing agents. Such methods are disclosed in for example US2010143591, US2009142482, US20060264518 and US20080220155, EP-As 2147733, 2139007, 803551, 2012952, 2030706, 1683592, 166617, 2119747, 2087490 and 2010314, WOs 2008/151066, 2006/076603, 2009/152388 and 2009/157393.
Among others, the dilution of metallic nanodispersions, usually less than 1 wt. % of metallic particles, is a severe drawback. Indeed, such highly diluted metallic nanodispersions cannot directly be used to prepare a conductive coating or a printing fluid that requires at least 5 wt. % of metallic nanoparticles based on its composition. An additional concentration step of the diluted metallic nanodispersions is then necessary before it can be used in the preparation of such coating or printing fluids.
WO2006/072959 discloses the production of silver nanoparticles dispersions up to 35 wt. % in water but the method still requires additional purification and isolation steps that impart drastically their industrialization and the scope of their applications.
A metallic nanodispersion typically comprises metallic, metal oxide or metal precursor nanoparticles, a polymeric dispersant and optionally a dispersion medium. The polymeric dispersant is a substance that promotes the formation and stabilization of a dispersion of particles in a dispersion medium. Dispersed particles may have a tendency to re-agglomerate after the dispersing operation, due to mutual attraction forces. The use of dispersants counteracts this re-agglomeration tendency of the particles. The dispersant has to meet particularly high requirements when used for coating fluids and printing inks. Non-stable dispersions may lead to irreversible phase separation causing among other the clogging of the coating or print heads, which are usually only a few micrometers in diameter. In addition, metallic particles agglomeration and the associated blockage of the coating/print heads has to be avoided in the standby periods of the system.
In the case of metallic nanoparticles dispersions, their tendency to re-agglomerate, to flocculate or to precipitate (leading to phase separation) is enhanced due to their high bulk density (ρ) when compared to other organic or inorganic particles like organic pigments (ρ=1.2-2.0 g/cm3), inorganic pigments (ρ=4.2 g/cm3 for titanium dioxide) or inorganic filler (ρ=4.4 g/cm3 for barium sulphate). For example, the bulk densities at room temperature of silver, cupper and gold are respectively 10.49, 8.94 and 19.30 g/cm3.
Polymeric dispersants typically contain in one part of the molecule so-called anchor groups, which adsorb onto the metallic particles to be dispersed. In a spatially separate part of the molecule, polymeric dispersants have polymer chains compatible with the dispersion medium (or liquid vehicle) and all the ingredients present in the final coating or printing fluids. Typical polymeric dispersants include homopolymer or random or block copolymers of various topologies and architectures (linear, graft, hyper branched).
Metallic nanoparticles dispersions usually comprise polymeric dispersants selected from homopolymers and copolymers based on acrylic acid, methacrylic acid, vinyl pyrrolidinone, vinyl butyral, vinyl acetate or vinyl alcohol.
EP-A 2147733 discloses a method for producing a silver ink from a dispersion containing a polymeric dispersant, the dispersant comprising a hydrophilic segment and a polyalkyleneimine chain.
US2009/0242854 describes compounds to improve the stability of metallic conductive inks, which comprise metallic nanoparticles, a polymeric dispersant and a solvent, wherein the respective boiling point may be less than 150° C. The polymers may have a head group and a tail group on a nanoparticle, wherein the head group may include compounds such as amines, cationic alkylammonium groups, carboxylic acids, sulfonic acids and phosphoric acid groups. Other additives may also be present in the dispersion, such as dimethyl sulfoxide and oxy-bis-ethanol. Compounds such as BYK-349, BYK-DYNWET800, isostearyl ethylimidazolinium ethosulphate and alcohols were added to some of the inks to promote good wetting properties and adhesion properties.
EP-A 10196244.7 (filed on 21-12-2010) describes a conductive ink comprising metallic nanoparticles and a polymeric dispersant which comprises an anchor group with affinity for the metallic nanoparticles, wherein the polymeric backbone has a 95 wt. % decomposition at a temperature below 300° C., thus allowing to be used in printing processes with lower curing temperatures.
The metallic nanodispersions are used to coat or print metallic patterns on a substrate. Typically, after applying the patterns on the substrate a sintering step, also referred to as curing step, at elevated temperatures is carried out to induce/enhance the conductive or the semi-conductive properties.
It has been found that the organic components of the nanodispersions, for example the polymeric dispersants, may reduce the sintering efficiency and thus the conductive or semi-conductive properties of a coated or printed surface. For this reason, higher sintering temperatures and longer sintering times are often required to decompose the organic components.
Typical polymeric dispersants, such as those described above, are characterized by a full decomposition temperature of at least 350° C. Therefore, the patterns coated or printed with the fluids or inks comprising such polymeric dispersants require a sintering step at elevated temperatures to be sure that most of the organic components in the coated or printed layers are decomposed.
Such high sintering temperatures are not compatible with common polymer foils, such as polyethylene terephthalate (PET) or polycarbonate, which have relatively low glass transition temperatures. This restricts the choice to more expensive polymers such as polyimide.
In order to enlarge the application of metallic, metal oxide or metal precursor nanoparticles dispersion to thermo-sensitive substrates it is desirable to obtain metallic, metal oxide or metal precursor compositions which require lower temperatures and times for the curing step, with good or improved conductivity values. Additionally, it is also desirable to develop a curing process to take advantage of the improved properties of the compositions, i.e. a high throughput coating or printing process.